It is well known that exposure to sunlight can pose a number of hazards to the skin. These damaging effects may result not only from sunbathing but also from the sunlight exposure associated with daily outdoor activities. The major short term hazard of prolonged exposure to sunlight is erythema, i.e. sunburn, which results from UVB radiation having a wavelength of from about 290 nm to about 320 nm. Over the long term, however, malignant changes in the skin surface often occur. Numerous epideminologic studies demonstrate a strong relationship between sunlight exposure and human skin cancer. Another long term hazard of ultraviolet radiation is premature aging of the skin, which is primarily caused by UVA radiation having a wavelength of from about 320 nm to about 400 nm. This condition is characterized by wrinkling and pigment changes of the skin, along with other physical changes such as cracking, telangiectasis, solar dermatoses, ecchymoses, and loss of elasticity. The adverse effects associated with exposure to UV radiation are more fully discussed in DeSimone, "Sunscreen and Suntan Products," Handbook of Nonprescription Drugs, 7th Ed., Chapter 26, pp. 499-511 (American Pharmaceutical Association, Washington, D.C.; 1982); Grove and Forbes, "A Method for Evaluating the Photoprotection Action of Sunscreen Agents Against UV-A Radiation," International Journal of Cosmetic Science, 4, pp. 15-24 (1982); and U.S. Pat. No. 4,387,089, DePolo, issued Jun. 7, 1983.
As a result of the abovementioned hazards typically associated with sunlight exposure, the general public's interest in the sun protection product market has grown considerably. In today's commercial market, there are not only sunscreen products for sunbathing but there are also a variety of personal care products containing sunscreens, particularly cosmetic type products which are worn daily. "Personal care products" refer to health and cosmetic beauty aid products generally recognized as being formulated for beautifying and grooming the skin and hair. For example, personal care products include sunscreen products (e.g., lotions, skin creams, etc.), cosmetics, toiletries, and over-the-counter pharmaceutical products intended for topical usage.
A wide variety of sunscreen actives have been used in personal care products. It is desirable that the sunscreen active or active system provide broad spectrum UV protection, i.e., protection against both UVA radiation and UVB radiation. It is further desirable that the sunscreen actives are formulated to provide stable, efficacious, and aesthetically appealing sunscreen products. In addition, for economic reasons it is often desirable that the sunscreen actives be approved for global use. Dibenzoylmethane compounds are one class of sunscreen compounds which provide broad spectrum UV protection and are approved for global use.
In personal care products, sunscreen actives are often formulated with water, since the resulting systems tend to be more aesthetically appealing to the user. The presence of water, however, promotes the growth of microorganisms which can cause deterioration of the sunscreen product. This deterioration can undesirably alter a product, rendering it physically or chemically unaesthetic or shortening the product's useful life. In some instances, such deteriorated sunscreen products could even be injurious to humans. Therefore, sunscreen products are generally designed in an effort to minimize deterioration over the product's desired shelf life. Antimicrobial preservatives are typically included in aqueous sunscreen products to prolong their useful life during storage and usage and to maintain product efficacy (i.e., UV absorbance).
Antimicrobial preservatives should be non-irritating to the skin, effective against microorganisms typically found in aqueous sunscreen products which pose the greatest threat to the consumer, and/or which tend to cause sunscreen product instability (i.e., chemical side reactions and consequential product inefficacy), cost efficient, and easily formulated.
A wide variety of antimicrobial preservatives are commonly used in personal care products, including the parabens, isothizolinones, formaldehyde-donating preservatives such as hydantoins, alcohols, and others such as described in the Preservatives Documentary/Encyclopedia issue of Cosmet. & Toilet., Vol. 102, No. 12 (1987). However, the formulation of aqueous systems containing dibenzoylmethane sunscreen actives is not straightforward. Formaldehyde donor preservatives, in particular, have been known to interact with dibenzoylmethanes. While not intending to be limited by theory, it is believed that an aldol reaction product is formed when dibenzoylmethanes are combined with formaldehyde-donating preservatives. Over time, this interaction tends to result in poor storage stability, i.e., increased microbial growth in the product and/or the loss of UV absorbance capability of the dibenzoylmethane sunscreen.
It has surprisingly been found that compositions which include an emulsion having at least one oil phase comprising a dibenzoylmethane sunscreen active and at least one aqueous phase comprising a formaldehyde-donating preservative exhibit excellent storage stability and retention of UV absorbance capability. The growth of microorganisms in the product and therefore spoilage of the product is effectively prevented, inhibited or retarded such that the product life is extended. It has also been found that these compositions provide a means of delivering the dibenzoylmethane sunscreen active to the skin in a non-irritating manner.